# How are water vapors not visible?

This site says that water vapor isn't visible.

However, take a look at this picture:

Isn't that water vapor?

• boil a small amount of water in a tall beaker, or glass jar, with a loose lid in a microwave oven , once you get it up to temperature the water vapour filling the jar will be invisible. – Jasen Jan 9 '17 at 2:56
• True story about invisible water vapor. In many power plants today, steam is used to drive turbines (heated by various means). In the case of a steam leak, the steam is at tremendously high pressure and temperature. This means fast streams of steam that don't start to form visible condensation for 15-20 feet, or more. It's also terribly noisy, so you can't hear where the leak is. When looking for these leaks, one waves a broom handle in front of them. They know when they found the leak because the steam will literally cut the broom handle in half! That is invisible water vapor! – Cort Ammon Jan 9 '17 at 4:12
• It always bugs me when people talk about clouds being made of water vapor. No, they not - if you can see them, they're not water vapor, because water vapor is invisible. – Nuclear Hoagie Jan 9 '17 at 15:22
• In this experiment on lighting a match with water you will see the difference between water the mist from water heated to the boiling point and water vapor heated to 200 degrees. – kasperd Jan 10 '17 at 8:31
• @CortAmmon please please please tell me there's a video of this happening somewhere! – BruceWayne Jan 11 '17 at 6:41

Water vapour is a clear and colourless gas, so it can't be seen by the naked eye.

What you see in the photo in your second link is (partially) condensed water vapour, i.e. fog (or mist). Fog contains tiny, discrete water droplets and light bounces off their surface in random directions, causing the visibility.

Water vapour by contrast only contain free molecules, too small for light to bounce off, so pure water vapour (without any condensate) is invisible, like most gases (some gases are clear but coloured like chlorine gas).

• Comments are not for extended discussion; this conversation has been moved to chat. – rob Jan 9 '17 at 15:15
• Gert, when you say light bounces on water droplets, is it all light wavelength? I mean, visible spectrum clearly bounces of, but does IR also scatter on water droplets? – Pedro Batista May 12 '17 at 11:26

Trouble is caused by definitions of steam and vapour in physics and in common language.

Physical definition of water vapour and steam is gaseous phase of water. In common language it is "the white cloud above pot with warm water in it when it is cold there".

Take glass kettle and put it on a stove and boil the water in it.

You will see bubbles growing in the bottom and disappearing in the water volume. Those bublles contain water vapour only. If you reach boiling temperature in whole volume, such bublles stop vanishing and will run to the top where they pop.

You will also see that those bubbles are colourless instead of being white or grey. That means they are filled with (invisible) colourless gas - water vapour.

Once they pop, the vapour leaves hot water surrounding and enter colder air where it dissolves until it reaches saturation point and no more water vapour can dissolve - so it condensates. Above the kettle it forms "steam", outside it forms fog and clouds. It condensate on any interface it can - lid, dust, water droplet - and form humid air - water droplets mixture. Both phases are colourless but because of different indices fo refraction the light is shattered in there and you can see white (grey) cloud of steam.

Observing the steam raising from the kettle you can see it vanishes. It is caused by high-temperature droplets reaching unsatureated air, evaporating again and forming one colourless phase - humid air.

The bubble forms when (small ammount of) liquid water goes gaseous, the bubble contain water only. There are no gases - there is only water vapour in it. When the bubble pops, the vapour in it will be released to the mixture of gases called air and will difuse in it (increasing absolute humidity). If it was not water but liquid chlorine, chlorine gass will diffuse instead (increasing its concentration in the air). If it was a block of dry ice, carbon dioxide gas is released and dissolves in the air.

When the temperature is below the dew point (driven by humidity and pressure) the water content in the (over)saturated air will condensate. In case of boiling water, the humidity changes (increases) and thus the water in the air condensate and form the white opaque steam cloud. In case of chlorine, you will see yellow transparent cloud. In case of dry ice the heat consumed to warming the block and evaporating the carbon dioxide causes decrease of temperature of the humid air in the surroundings. When the temperature drops below dew point, water will condensate and you will see white, opaque cloud.

The steam cloud is white because water does not absorb light of specific wavelength (colour). It is opaque because of large ammount of small particles that are round or randomly oriented. Thus they are reflecting light to random directions. Cloud made of copper sulfate, $CuSO_4\times5H_2O$, dust will be blue (because of selective light absorbtion) and opaque (because of light scattering).

Suppose we have small volume of hot water vapour only surrounded by cold humid air.

• The temperature will equalize through heat dissipation.
• Water vapour will diffuse in the humid air.

What you will observe strongly depends on tempreature(s), pressure and volumes. If the air humidity is low enough, the water will dissolve without any effects. If the humidity is large enough, the temperature in water vapour and close the border may drop below dew point and because of oversaturated state the vapour may condensate and release another heat (condensation enthalpy). This will form the visible cloud. The water droplets then diffuse in the air and may evaporate (and consume same ammount of heat as it was released during condensation) or dissipate in the volume. In both cases the cloud will become indistiguishable form the rest of the volume.

• So these gases become bubbles and when they pop they become water vapors? and then these "hot invisible" vapours of water mix with air and form a fog? and because water is a shiny surface light scatters off it, and we can see the fog? But I have one question though, what happens if there was a little amount of hot water vapor mixed with cold air? Would the small amount still be able to be shown no matter what the amount of small water is mixed since light can scatter water? – Okama Ksakas Jan 9 '17 at 13:12
• No, bubbles are filled with water vapour only. When they pop, vapour is released to the air. – Crowley Jan 9 '17 at 13:13
• That's what I mean. My other questions still apply though..Can you please answer them? Thanks. – Okama Ksakas Jan 9 '17 at 13:14
• I suggest exchanging the copper sulfate for a substance that can actually evaporate and has colored vapour, e.g. iodine I2. Copper sulfate is a salt that decomposes at high temp. Also, the blue color of CuSO4·5H2O comes from hydration of the $Cu^{2+}$, the anhydrate CuSO4 is colorless. In general, it is not a very safe bet to extrapolate the color of a liquid (let alone a solution) to the color of the gas (other than there are plenty of substances that are colorless to the human eye in both liquid and gaseous form). – cbeleites unhappy with SX Jan 10 '17 at 18:18
• @cbeleites Good points, but i thought of $CuSO_4\cdot5H_2O$ dust rather than gas. The case of coloured gas is mentioned with chlorine. – Crowley Jan 11 '17 at 9:49

Your question is partly a language question. Natural language is terribly imprecise and ambiguous. This is the underlying reason for all sciences to develop their own terminology.

In this case two of the four meanings Merriam-Webster lists for vapor are unfortunately almost the opposites of each other.

Concretely:

• Yes, the white clouds you see are "vapor" in everyday language, as every adult is happy to point out to a three-year-old. This is the first definition in Merriam-Webster's entry on vapor:

Diffused matter (as smoke or fog) suspended floating in the air and impairing its transparency.

This "vapor" is a fog, just like clouds: Tiny drops of fluid suspended floating in the air.

• No, what you see is emphatically not "vapor" in the scientific sense. Scientifically, vapor is defined as

a substance in the gaseous state as distinguished from the liquid or solid state

as the second definition for vapor in Merriam-Webster's states. In order to know what a gas is we can look at its Wikipedia article:

A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or compound molecules made from a variety of atoms (e.g. carbon dioxide).

Water vapor, i.e. water in the gaseous state, consequently consists of individual water molecules which do not reflect or (at this scale) significantly absorb or disperse visible light due to their small size. Because the visible fog, or the clouds, above boiling water, which are called "vapor" in everyday language, consist of little droplets of fluid (not gaseous!) water, they are not "vapor" in this scientific sense. Paradoxically, water vapor in the scientific sense is exactly what you don't see in the picture.

• +1 for "language is terribly precise and ambiguous". that's why we have tools like lambda calculus and math. and lawyers. – Walrus the Cat Jan 9 '17 at 19:18
• @WalrustheCat ITYM imprecise. – Jens Jan 11 '17 at 10:02
• haha yes. and prone to user-error. – Walrus the Cat Jan 12 '17 at 22:30

Pretty much every gas can be condensed into a liquid at some temperature, and pretty much every liquid can be "boiled off" as a gas . It's just that some go through this transformation at a high temperature and others at a low temperature.

Water's transition point (at sea-level pressure) is 100 degrees C, so you mostly experience it as a liquid. Oxygen, on the other hand, boils at −183°C, so you only see the liquid when it's been condensed with special equipment.

But below its boiling point, water still evaporates into the air in relatively small amounts, until the "saturation point" is reached for a given temperature and pressure. (This is what weathermen are talking about when they talk about "percent humidity".) When water is near the saturation point (near "100 percent humidity") it switches back and forth between liquid and gas, and so tiny droplets of water can end up suspended in mid-air. It is these tiny droplets that you see in fog, clouds, the "steam" arising out of a tea kettle, etc.

※ I say "pretty much" because some materials may go through a chemical transformation of some sort before they reach their "boiling point".

• Doesn't pressure mean force? I don't understand what does "for a given pressure(force)'' mean? how does the pressure affect saturation point? I know that at a specific temperation, like 100C would be water boiling. And saturation point is percent humidity is what I understood from you. How does waer suspend in mid air, does't suspend mean mix? – Okama Ksakas Jan 9 '17 at 8:50
• Well, near sea shore hte watter boils at 100°C. In Himalaya, 8km above sea level, water boils at much lower temperature. Saturation point means 100% relative humidity - you cannot add more water vapour into the gas phase. If you have RH 50% it means that the air contains half of its water vapour capacity at given temperature and pressure. The higher temperature the higher capacity, the lower pressure, the higher capacity. – Crowley Jan 9 '17 at 11:47
• Ah so what I understood is that, percent humidity is a point reached where the gas becomes liquid again and liquid becomes gas, which means a mixture of condensation and evaporation, right? and How does it switch back and forth between liquid and gas? Is it a set rule as percent humidity?I also don't understand how suspend(mixing) water with the air happen? do you mean the water vapors? Or is there something wrong with my definition of suspend.. I am sorry if there is, the definition does look a lot complicated on google than it really is. – Okama Ksakas Jan 9 '17 at 12:13
• @OkamaKsakas - Think about it -- when some gaseous water in the air condenses, because conditions are hovering around 100% humidity, it will first condense into tiny droplets of water. When this happens, the water will give off a tiny amount of heat, and so the relative humidity (which depends in part on temperature) will be driven down. So you're left with tiny droplets of water in the air, switching back and forth. Eventually gravity will pull them to the ground, but random air movements (some from the heat given off) will tend to keep the droplets suspended. Think of "Brownian motion". – Hot Licks Jan 9 '17 at 12:53
• @OkamaKsakas This is true of any thermodynamic process, really - it's always a two-way street, and we just tweak which side gets the upper hand. For example, we can dissolve sugar in hot water up to the point it can't dissolve any more - the solution is saturated. But what's really happening is that the individual "molecules" keep going in and out of the solution at the same rate. When you make the water colder, the "get in solution" it gets slower, and the sugar starts precipitating out of the solution in bulk, forming crystals. – Luaan Jan 9 '17 at 14:41

When water molecule changed to vapor at ideal environment it's invisible by naked eye. At condensation state molecules reflect light rays by bending principle, therefore we can observe this vapor condensation like veil smoke effect